Superhard constructions &amp; methods of making same

ABSTRACT

A superhard polycrystalline construction comprises a body of polycrystalline superhard material comprising a first superhard phase having a first average grain size; and a second superhard phase having a second average grain size. The second superhard phase is located in one or more channels or apertures in the first superhard phase, the first superhard phase forming a skeleton in the body of superhard material. The second superhard phase is bonded to the first superhard phase by a non-superhard phase and the first superhard phase differs from the second superhard phase in average grain size and/or composition. There is also disclosed a method of making such a superhard polycrystalline construction.

FIELD

This disclosure relates to superhard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of superhard tool inserts may be limited by fracture of the superhard material, including by spalling and chipping, or by wear of the tool insert.

Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and a superhard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the superhard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond.

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive or ultra hard material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering.

Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with a superhard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline superhard diamond or polycrystalline CBN layer.

In some instances, the substrate may be fully cured prior to attachment to the superhard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the superhard material layer.

Ever increasing drives for improved productivity in the earth boring field place ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements or tool inserts comprising PCD material are widely used in drill bits for boring into the earth in the oil and gas drilling industry. Rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite.

The most wear resistant grades of PCD usually suffer from a catastrophic fracture of the cutter before it has worn out. During the use of these cutters, cracks grow until they reach a critical length at which catastrophic failure can occur, namely when a large portion of the PCD breaks away in a brittle manner. These long, fast growing cracks encountered during use of conventionally sintered PCD, result in short tool life.

Furthermore, despite their high strength, polycrystalline diamond (PCD) materials are usually susceptible to impact fracture due to their low fracture toughness. Improving fracture toughness without adversely affecting the material's high strength and abrasion resistance which are critical for the material's ability to cut through rock, for example, is a challenging task.

There is therefore a need for a PCD composite that has good or improved abrasion resistance, fracture and impact resistance and a method of forming such composites.

SUMMARY

Viewed from a first aspect there is provided a superhard polycrystalline construction comprising:

a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising:

a first superhard phase having a first average grain size; and

a second superhard phase having a second average grain size;

wherein the second superhard phase is located in one or more channels or apertures in the first superhard phase, the first superhard phase forming a skeleton in the body of superhard material, the second superhard phase being and bonded thereto the first superhard phase by a non-superhard phase; and

wherein the first superhard phase differs from the second superhard phase in average grain size and/or composition.

Viewed from a second aspect there is provided a method of forming a superhard polycrystalline construction, comprising:

providing a first mass of particles or grains of superhard material for forming a first superhard phase; sintering the first superhard phase and forming a skeleton having a plurality of channels and/or apertures therein;

providing a second mass of superhard grains or particles for forming a second superhard phase;

positioning the second mass of superhard grains or particles in one or more channels and/or apertures in the skeleton formed of the first superhard phase to form a pre-sinter assembly; wherein the first superhard phase differs from the second superhard phase in average grain size and/or composition; and

treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 5.5 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, wherein the body of polycrystalline superhard material comprises a working surface, the working surface being formed of alternating portions of the skeleton and the second superhard phase located in the plurality of channels and/or apertures in the skeleton.

Viewed from a further aspect there is provided a tool comprising the superhard polycrystalline construction defined above, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

The tool may comprise, for example, a drill bit for earth boring or rock drilling, a rotary fixed-cutter bit for use in the oil and gas drilling industry, or a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

Viewed from another aspect there is provided a drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of an example PCD cutter element for a drill bit for boring into the earth;

FIG. 2 is a plan view of an example of a PCD cutter element;

FIG. 3 is a schematic flow diagram of the process of forming an example PCD cutter element such as that shown in FIG. 2;

FIG. 4 is a schematic partial cross-section through a further example of a PCD cutter element;

FIG. 5a is a schematic partial cross-section through a still further example of a PCD cutter element;

FIG. 5b is a plan view of a further example of a PCD cutter element; and

FIG. 6 is a plot showing the results of a vertical borer test comparing two conventional PCD cutters with differing average diamond grain sizes with the PCD cutter element shown in FIG. 2.

The same reference numerals refer to the same general features in all the drawings.

DESCRIPTION

As used herein, a “superhard material” is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of superhard materials.

As used herein, a “superhard construction” means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In one embodiment of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, “interstices” or “interstitial regions” are regions between the diamond grains of PCD material. In embodiments of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

A “catalyst material” for a superhard material is capable of promoting the growth or sintering of the superhard material.

The term “substrate” as used herein means any substrate over which the superhard material layer is formed. For example, a “substrate” as used herein may be a transition layer formed over another substrate.

As used herein, the term “integrally formed” regions or parts are produced contiguous with each other and are not separated by a different kind of material.

In an embodiment as shown in FIG. 1, a cutting element 1 includes a substrate 10 with a layer of superhard material 12 formed on the substrate 10. The substrate 10 may be formed of a hard material such as cemented tungsten carbide. The superhard material 12 may be, for example, polycrystalline diamond (PCD), or a thermally stable product such as thermally stable PCD (TSP). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown) and may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

The exposed top surface of the superhard material opposite the substrate forms the cutting face 14, which is the surface which, along with its edge 16, performs the cutting in use.

At one end of the substrate 10 is an interface surface 18 that forms an interface with the superhard material layer 12 which is attached thereto at this interface surface. As shown in the embodiment of FIG. 1, the substrate 10 is generally cylindrical and has a peripheral surface 20 and a peripheral top edge 22.

As used herein, a PCD grade is a PCD material characterised in terms of the volume content and size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. A grade of PCD material may be made by a process including providing an aggregate mass of diamond grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for diamond to a pressure and temperature at which diamond is more thermodynamically stable than graphite and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a PCD structure. The aggregate mass may comprise loose diamond grains or diamond grains held together by a binder material and said diamond grains may be natural or synthesised diamond grains.

Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K₁C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

All of the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The PCD structure 12 may comprise one or more PCD grades.

FIG. 2 is a plan view of an embodiment of PCD material forming the super hard layer 12 of FIG. 1. The superhard layer 12 comprises a first phase of superhard material forming a skeleton or framework 100 which, in the example in FIG. 2, is in the form of a spoked disc or section with spokes extending from a central hub section, and a second superhard phase 120 located between adjacent spokes.

The superhard material of the first and second phases 100, 200 may be comprised of inter-bonded grains of superhard material such as, for example, diamond grains or particles. In each phase, the starting mixture prior to sintering may be unimodal or multimodal, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains which are to form one or more of the alternating layers or strata. In some embodiments, the coarse fraction may have, for example, an average particle/grain size ranging from about 10 to 60 microns. By “average particle or grain size” it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the “average”. The average particle/grain size of the fine fraction is less than the size of the coarse fraction, for example between around 1/10 to 6/10 of the size of the coarse fraction, and may, in some embodiments, range for example between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse diamond fraction to the fine diamond fraction ranges from about 50% to about 97% coarse diamond and the weight ratio of the fine diamond fraction may be from about 3% to about 50%. In other embodiments, the weight ratio of the coarse fraction to the fine fraction will range from about 70:30 to about 90:10.

In further embodiments, the weight ratio of the coarse fraction to the fine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse and fine fractions do not overlap and in some embodiments the different size components of the compact are separated by an order of magnitude between the separate size fractions making up the multimodal distribution.

Some embodiments may comprise a wide bi-modal size distribution between the coarse and fine fractions of superhard material, but some embodiments may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude, for example, a blend of particle sizes whose average particle size is 20 microns, 2 microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In embodiments where the superhard material is polycrystalline diamond material, the diamond grains used to form the polycrystalline diamond material may be natural or synthetic.

In some embodiments, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some embodiments, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some embodiments, the metal carbide is tungsten carbide.

The cutter of FIG. 1 may be fabricated, for example, as shown in the flow diagram of FIG. 3.

As used herein, a “green body” is a body comprising grains to be sintered and a means of holding the grains together, such as a binder, for example an organic binder.

The mesostructure of the PCD element 200 shown as being prepared in FIG. 3 is comprised of two PCD phases, the first phase having a first diamond particle size distribution and the second phase having a different diamond particle size distribution. As described above, the different particle size distributions may in turn be unimodal, comprising a single grade of diamond grains, or multimodal comprising two or more different grades of diamond grains or particles. Each material grade powder is prepared separately by ball milling to produce the particle size distribution of interest. The individual powders may be mixed by ball milling with a catalyst binder such as, for example, Co, Ni, Fe, Mn, Pt, and Ir and/or combinations thereof. In some embodiments, no additional catalyst binder is included in this manner.

As shown in FIG. 3, in a first stage, diamond powder of one diamond grain size grade is sintered by cobalt infiltration from a WC substrate at high pressure, preferably above 5 GPa, and high temperature, preferably above 1400 deg Celsius.

In another embodiment, powder of one grade of diamond may be sintered as a solid PCD without infiltration. This is achievable by, for example, third-generation admixing.

One or more discs of sintered PCD which are to form the skeleton or framework in the PCD table 12 may be prepared as above. The disc(s) is/are then polished and engineered slots or apertures are cut in the PCD disc(s) to produce the desired skeleton mesostructure or framework 200 using, for example, an EDM process, laser abrasion or ablation, or a die sinking process.

The slots or apertures in the skeleton 200 may take any shape, as desired for the particular application, such as, for example, circular, square, rectangular, or polygonal or a mixture thereof.

In the second stage shown in FIG. 3, the skeleton 200 is introduced into a niobium cup. A powder mix of the second phase material 300 such as diamond powder, diamond shredded paper, or, for example, diamond slurry in inert liquid, is placed into the cup to fill the open volumes in the skeleton 200 and to form an interface with a substrate 320 which is placed on top of the assembly to form a pre-composite. The substrate 320 may be, for example, a composite of WC, or alumina, and may include a sintering catalyst such as Co, Ni, Fe, or Mn, for example, which will infiltrate the skeleton during HPHT sintering.

The pre-composite is then consolidated to increase the green body density by methods such as vibration compaction, cold isostatic pressing, or HIP. In some embodiments, the binder materials may be removed from the pre-composites by heat treatment at 650 C in an 5% H2/N2 atmosphere.

The pre-composites may then be outgassed at, for example, 1050 C under vacuum (10⁻⁵ mbar).

The pre-composite is then sintered in an HPHT process at a temperature of around 1400 C and a pressure of, for example, greater than 5 GPa to form a PCD compact such as that shown in FIG. 1.

In an alternative embodiment, a slurry of diamond or superhard material powder is prepared in a mixture of alcohol such as methanol or ethanol and a plasticizer such as DBP. The slurry is then homogenised in a tubular mixer. A paper of diamond or superhard material is prepared by casting on a moving table and dried at about 60 C. The paper thickness may be, for example, 200 micrometres or less. A male structure reproducing the desired mesostructure is formed on a solid punch, made of, for example, WC, hardened steel or any high strength material. The punch is used to create the open spaces/volumes in individual papers which are to form the mesostructure. A number of individual perforated papers are stacked together to produce a skeleton mesostructure of required thickness. The papers are then stacked in a niobium cup and the method described above for filling the voids in the mesostructure and sintering to form the PCD compact are followed.

In a further alternative embodiment for producing the skeleton, a green body skeleton of one diamond grade, with a desired mesostructure, is injection moulded or 3D printed using an appropriate binder. The open volumes in the skeleton may take any shape, such as, for example, circular, square, rectangular, or polygonal, or any desired combination thereof. The skeleton is then placed in a niobium cup and the method described above for filling the voids in the mesostructure and sintering to form the PCD compact are followed.

In another embodiment the skeleton may be formed as follows. A male green body skeleton of one diamond graded, with a desired mesostructure, is injection moulded or 3D printed using an appropriate binder. A female green body skeleton of one diamond grade, with a desired mesostructure, is injection moulded or 3D printed using an appropriate binder. The open volumes in the skeleton may take any shape, such as circular, square, rectangular, and polygonal or any combination thereof.

The male and female parts are assembled, placed on top of a WC-catalyst substrate in a niobium cup as described in the methods above and the pre-composite is sintered at HPHT, for example at a temperature of above 1400 C and pressure above 5 GPa.

In yet another alternative method, the skeleton may be prepared as follows. A green body with a desired mesostructure consisting of alternating material phases is 3D printed using an appropriate binder. Alternating material phases may be, for example, PCD of different grades, PCD and an oxide or ceramic or WC or any other hard metal. The green body is placed on top of a pre-formed WC-catalyst substrate and sintered at HPHT as described above.

In the embodiment where the skeleton or framework 100, 200 is pre-sintered, the skeleton may be subjected to a treatment such as acid leaching, to remove residual catalyst/binder from some or substantially all of the interstices between the inter-bonded diamond grains to reduce the catalyst content therein. This is prior to the step of subjecting the skeleton to a second HPHT sintering cycle in which the volumes in the voids or channels in the skeleton are filled with the second superhard phase. Thus the skeleton goes through two HPHT sintering cycles (referred to as double-sintering).

The starting skeleton disc 100, 200 may be made of a more abrasive and high impact resistance PCD grade (or material) 300 than that used to fill the voids or channels in the skeleton, or vice-versa, as desired depending on the intended application of the sintered PCD compact, the empty volumes of the skeleton disc being filled with, for example, a diamond powder grade different in composition and/or particle size from that used in the starting skeleton green body to achieve the desired construction.

As described above, the starting skeleton disc may also be prepared as a green body using 3D printing or injection moulding. In this case, the insert only goes through one HPHT sintering cycle.

As described above, the skeleton 100, 200 may be formed of one or more layers or stacked discs having any desired combination of voids or channels formed therein, aligned in a particular chosen configuration. FIGS. 2, 4, 5 a and 5 b show alternative configurations for the skeleton, the configuration shown in FIG. 2 being that of a spoked structure such that, in the final sintered product, the body of superhard material comprises alternating sectors which may be, for example, concentric vertical layers as shown in FIG. 2, which may, in other embodiments, be inclined with respect to the vertical axis as shown in FIG. 4 or regions as shown in FIGS. 5a an 5 b. The alternating sectors, layers or regions are bonded together by infiltration and reaction with a catalyst material during a high pressure high temperature sintering.

As described above, embodiments of either the skeleton 100, 200 and/or the second superhard phase 300 filling the voids or channels in the skeleton may be made by a number methods of preparing a green body. The green body or bodies comprise(s) grains or particles of superhard material, and a binder, such as an organic binder. The green body or bodies may also comprise catalyst material for promoting the sintering of the superhard grains. The green body or bodies may be made by combining the grains or particles with the binder and forming them into a body having substantially the same general shape as that of the intended sintered body, whether that be the skeleton or second superhard phase which is to fill the channels or voids in the skeleton, and drying the binder. At least some of the binder material may be removed by, for example, burning it off. The green body may be formed by a method including a compaction process, injection or other methods such as molding, extrusion, deposition modelling methods.

The substrate 320 may provide a source of catalyst material for promoting the sintering of the superhard grains. In some embodiments, the superhard grains may be diamond grains and the substrate may be cobalt-cemented tungsten carbide, the cobalt in the substrate being a source of catalyst for sintering the diamond grains. The pre-sinter assembly may comprise an additional source of catalyst material.

After sintering, the polycrystalline super hard constructions may be ground to size and may include, if desired, a 45° chamfer of approximately 0.4 mm height on the body of polycrystalline super hard material so produced.

In embodiments where the cemented carbide substrate 320 does not contain sufficient solvent/catalyst for diamond, and where the PCD structure is integrally formed onto the substrate during sintering at an ultra-high pressure, solvent/catalyst material may be included or introduced into the aggregated mass of diamond grains from a source of the material other than the cemented carbide substrate. The solvent/catalyst material may comprise cobalt that infiltrates from the substrate in to the aggregated mass of diamond grains just prior to and during the sintering step at an ultra-high pressure. However, in embodiments where the content of cobalt or other solvent/catalyst material in the substrate is low, particularly when it is less than about 11 weight percent of the cemented carbide material, then an alternative source may need to be provided in order to ensure good sintering of the aggregated mass to form PCD.

Solvent/catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent/catalyst material in powder form with the diamond grains, depositing solvent/catalyst material onto surfaces of the diamond grains, or infiltrating solvent/catalyst material into the aggregated mass from a source of the material other than the substrate, either prior to the sintering step or as part of the sintering step. Methods of depositing solvent/catalyst for diamond, such as cobalt, onto surfaces of diamond grains are well known in the art, and include chemical vapour deposition (CVD), physical vapour deposition (PVD), sputter coating, electrochemical methods, electroless coating methods and atomic layer deposition (ALD). It will be appreciated that the advantages and disadvantages of each depend on the nature of the sintering aid material and coating structure to be deposited, and on characteristics of the grain.

In one embodiment, the bonder/catalyst such as cobalt may be deposited onto surfaces of the diamond grains by first depositing a pre-cursor material and then converting the precursor material to a material that comprises elemental metallic cobalt. For example, in the first step cobalt carbonate may be deposited on the diamond grain surfaces using the following reaction:

Co(NO₃)₂+Na₂CO₃CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or other solvent/catalyst for diamond may be achieved by means of a method described in PCT patent publication number WO/2006/032982. The cobalt carbonate may then be converted into cobalt and water, for example, by means of pyrolysis reactions such as the following:

CoCO₃->COO+CO₂

COO+H₂->CO+H₂O

In another embodiment, cobalt powder or precursor to cobalt, such as cobalt carbonate, may be blended with the diamond grains. Where a precursor to a solvent/catalyst such as cobalt is used, it may be necessary to heat treat the material in order to effect a reaction to produce the solvent/catalyst material in elemental form before sintering the aggregated mass.

In some embodiments, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate. The binder material may comprise between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprises Co.

A PCD compact according to an embodiment comprising a skeleton or framework 100, 200 formed of PCD having an average diamond grain size of around 4 microns and voids filled with PCD 300 having an average diamond grain sixe of around 22 microns was compared in a vertical boring mill test with two conventional PCD cutters formed of diamond having an average grain size of around 4 microns (FG302) and two PCD cutters formed of diamond having an average grain size of around 22 microns (Quadmodal). The results are shown graphically in FIG. 6. The results of the test on the PCD embodiment is the middle line in FIG. 6. In this test, the wear flat area was measured as a function of the number of passes of the cutter element boring into the workpiece. The results provide an indication of the total wear scar area plotted against cutting length. It will be seen that the PCD compact formed according to an embodiment was able to achieve a greater cutting length and smaller wear scar area than that occurring in the conventional PCD formed of diamond having an average grain size of 22 microns, and a greater cutting length and similar wear scar to the conventional PCD compacts formed of diamonds having a fine grain size of around 4 microns. This means that a longer working life of the tool having an embodiment cutter is possible for similar wear scar formation.

Whilst not wishing to be bound by a particular theory, it is believed that the functionally graded PCD of embodiments, with alternating superabrasive phases between the skeleton which has been double-sintered and the superabrasive material in the voids or channels of the skeleton or framework, enables the combination of the high abrasion resistance of one material phase with the high impact resistance of the other resulting in a PCD material with a combination of good abrasion, fracture and impact resistance. Furthermore, it is believed that alternating the single sintered and double sintered boundaries by filling voids or channels in the skeleton which has been double-sintered with a superhard phase that has only been through a single sintering stage may assist in inhibiting the growth of flaws initiated in the first sintering process during the second sintering process, which could otherwise lead to cracks in use, and/or may assist in inhibiting the initiation of cracks during use. Also, the effect of thermal expansion of the skeleton or framework during the second sintering process is believed to be controlled by the presence of the unsintered second superhard phase which is first sintered during the second sintering phase. This is also believed to assist in inhibiting cracks from initiating during use as residual stresses in the PCD compact 10 may be favourably controlled.

The PCD elements 10 described with reference to FIGS. 1 and 2 may be processed by grinding to modify their shape. Also, catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be done by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be provided. The leaching depth in alternating sectors, layers or regions will be different due to different microstructures. This may be used to achieve a preferred leached profile.

Furthermore, the PCD body in the structure of FIGS. 1 and 2 comprising a PCD structure bonded to a cemented carbide support body may be created or finished to provide a PCD element which is substantially cylindrical and having a substantially planar working surface, or a generally domed, pointed, rounded conical or frusto-conical working surface. The PCD element may be suitable for use in, for example, a rotary shear (or drag) bit for boring into the earth, for a percussion drill bit or for a pick for mining or asphalt degradation.

While various embodiments have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed. For example, one or more different manufacturing methods may be used, including but not limited to EDM cutting and sintering of pre-sintered PCD inserts, injection moulding or 3D printing of green parts. The pre-sintered skeleton/perforated disc containing one PCD grade may be prepared by EDM cutting or laser abrasion or ablation and used in second stage sintering where a different PCD grade may be used to fill the empty volumes in the skeleton PCD disc. The result is a functionally graded PCD material with alternating PCD phases of different PCD grades. 

1. A superhard polycrystalline construction comprising: a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising: a first superhard phase having a first average grain size; and a second superhard phase having a second average grain size; wherein the second superhard phase is located in one or more channels or apertures in the first superhard phase, the first superhard phase forming a skeleton in the body of superhard material, the second superhard phase being bonded to the first superhard phase by a non-superhard phase; and wherein the first superhard phase differs from the second superhard phase in average grain size and/or composition.
 2. A superhard polycrystalline construction according to claim 1, wherein the superhard grains of the first and the second superhard phases comprise natural and/or synthetic diamond grains, the superhard polycrystalline construction forming a polycrystalline diamond construction.
 3. A superhard polycrystalline construction according to any one of the preceding claims, wherein the non-superhard phase comprises a binder phase.
 4. A superhard polycrystalline construction according to claim 3, wherein the binder phase comprises cobalt, and/or one or more other iron group elements, such as iron or nickel, or an alloy thereof, and/or one or more carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table.
 5. A superhard polycrystalline construction according to any one of the preceding claims, further comprising a cemented carbide substrate bonded to the body of polycrystalline material along an interface.
 6. A superhard polycrystalline construction according to claim 5, wherein the cemented carbide substrate comprises tungsten carbide particles bonded together by a binder material, the binder material comprising an alloy of Co, Ni and Cr.
 7. A superhard polycrystalline construction according to any one of claim 5 or 6, wherein the cemented carbide substrate comprises between around 8 to 13 weight or volume % binder material.
 8. A superhard polycrystalline construction according to any one of the preceding claims, wherein at least a portion of the body of superhard material is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.
 9. A superhard polycrystalline construction as claimed in claim 8, wherein the thermally stable region comprises at most 2 weight percent of catalyst material for diamond.
 10. A superhard polycrystalline construction according to any one of the preceding claims, wherein the skeleton formed of the first superhard phase comprises a perforated disc having said plurality of channels and/or apertures therein, the disc comprising a mass of interbonded superhard grains, the second superhard phase filling the channels and/or apertures in the disc.
 11. A superhard polycrystalline construction according to claim 10, wherein the skeleton formed of the first superhard phase comprises a plurality of stacked perforated discs.
 12. A superhard polycrystalline construction according to claim 11, wherein the discs are aligned such that the second superhard phase filling the channels and/or apertures form one or more of alternating sectors, concentric layers or regions, or layers or regions inclined with respect to the central longitudinal axis of the discs.
 13. A superhard polycrystalline construction according to any one of the preceding claims wherein the skeleton is pre-formed and has been subjected to two or more sintering processes.
 14. A superhard polycrystalline construction according to any one of the preceding claims wherein the skeleton comprises two or more channels and/or apertures therein, the body of polycrystalline superhard material comprising a working surface, the working surface being formed of alternating portions of the skeleton and the second superhard phase located in the two or more channels and/or apertures in the skeleton.
 15. A superhard polycrystalline construction for a rotary shear bit for boring into the earth, or for a percussion drill bit, comprising a superhard polycrystalline construction as claimed in any one of the preceding claims bonded to a cemented carbide support body.
 16. A method of forming a superhard polycrystalline construction, comprising: providing a first mass of particles or grains of superhard material for forming a first superhard phase; sintering the first superhard phase and forming a skeleton having a plurality of channels and/or apertures therein; providing a second mass of superhard grains or particles for forming a second superhard phase; positioning the second mass of superhard grains or particles in one or more channels and/or apertures in the skeleton formed of the first superhard phase to form a pre-sinter assembly; wherein the first superhard phase differs from the second superhard phase in average grain size and/or composition; and treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 5.5 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, wherein the body of polycrystalline superhard material comprises a working surface, the working surface being formed of alternating portions of the skeleton and the second superhard phase located in the plurality of channels and/or apertures in the skeleton.
 17. A method according to claim 16, wherein the step of providing a first mass of grains of superhard material and a second mass of superhard material comprises providing a first and second mass of diamond grains.
 18. A method according to claim 17, wherein the step of providing a first mass and a second mass of diamond grains comprises providing a first and/or a second mass of grains having a first fraction having a first average size and a second fraction having a second average size, the first fraction having an average grain size ranging from about 10 to 60 microns, and the second fraction having an average grain size less than the size of the coarse fraction.
 19. The method of claim 18, wherein the second fraction has an average grain size between around 1/10 to 6/10 of the size of the first fraction.
 20. The method of any one of claim 18 or 19, wherein the average grain size of the first fraction is between around 10 to 60 microns, and the average grain size of the second fraction is between about 0.1 to 20 microns.
 21. The method of any one of claims 18 to 20, wherein the weight ratio of the first fraction to the second fraction ranges from about 50% to about 97%, the weight ratio of the second fraction ranging from about 3% to about 50 weight %.
 22. The method of claim 18, wherein the ratio by weight percent of the first fraction to the second fraction is around 60:40.
 23. The method of claim 18, wherein the ratio by weight percent of the first fraction to the second fraction is around 70:30.
 24. The method of claim 18, wherein the ratio by weight percent of the first fraction to the second fraction is around 90:10.
 25. The method of claim 18, wherein the ratio by weight percent of the first fraction to the second fraction is around 80:20.
 26. The method of any one of claims 18 to 25, wherein the step of providing a mass of grains of superhard material comprises providing a mass of grains in which the grain size distributions of the first and second fractions do not overlap.
 27. The method of any one of claims 18 to 26, wherein the step of providing a mass of grains of superhard material comprises providing three or more grain size modes to form a multimodal mass of grains comprising a blend of grain sizes having associated average grain sizes.
 28. The method of claim 18, wherein the average grain sizes of the fractions is separated by an order of magnitude.
 29. The method of any one of claims 16 to 28, wherein the step of providing the first mass of superhard grains comprises sintering a first body of superhard grains; and forming the channels and/or apertures therein after sintering and prior to the step of positioning the second mass of superhard grains or particles in said channels and/or apertures.
 30. The method of claim 29, wherein the step of forming the channels and/or apertures comprises forming said apertures and/or channels using an EDM technique, or a laser ablation technique.
 31. The method of any one of claim 29 or 30, further comprising treating at least a portion of the sintered first mass of superhard grains to render said portion free of a catalyst material for the superhard grains, said portion forming a thermally stable region.
 32. The method of any one of claims 16 to 28, wherein the step of providing the first mass of superhard grains comprises forming a green body comprising the first mass of superhard particles or grains with said channels and/or apertures therein using one or more of 3D printing or injection molding techniques.
 33. The method of any one of claims 16 to 32, wherein the step of providing a first mass of superhard grains comprises providing a perforated disc forming the first mass having said a plurality of apertures and/or channels therein, the disc comprising a mass of interbonded superhard grains, the second superhard phase filling the apertures and/or channels in the disc.
 34. The method of claim 33, wherein the first superhard construction comprises a plurality of stacked perforated discs.
 35. The method of claim 34, further comprising aligning the discs such that the second superhard phase filling the apertures and/or channels forms one or more of alternating sectors, concentric layers or regions, or layers or regions inclined with respect to the central longitudinal axis of said discs, the step of sintering comprising bonding the discs together by infiltration and reaction with the non-superhard phase.
 36. A method according to any one of claims 16 to 35, further comprising providing a cemented carbide substrate for bonding to the body of polycrystalline material along an interface during the step of sintering.
 37. A tool comprising a superhard polycrystalline construction according to any one of claims 1 to 15, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.
 38. A tool according to claim 37, wherein the tool comprises a drill bit for earth boring or rock drilling.
 39. A tool according to claim 7 wherein the tool comprises a rotary fixed-cutter bit for use in the oil and gas drilling industry.
 40. A tool according to claim 37, wherein the tool is a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.
 41. A drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction according to any one of claims 1 to
 15. 42. A superhard polycrystalline construction substantially as hereinbefore described with reference to any one embodiment as that embodiment is illustrated in the accompanying drawings.
 43. A method of making a superhard polycrystalline construction, substantially as hereinbefore described with reference to any one embodiment as that embodiment is illustrated in the accompanying drawings. 